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The paper presents the transmission electron microscopy (TEM) investigations of the structural and phase conditions of the heat-affected zone of welded joint modified by plastic deformation. Tensile tests are carried out on INSTRON-1185 testing system at room temperature and 370 MPa loading. TEMinvestigations are carried out nearby the joint between the deposited and base metal, namely the type St3 steel. It is shown that the degree of plastic deformation has an effect on the material morphology, phase composition, defect structure and its parameters. It is shown that plastic deformation has no qualitative effect on the material structure, however it modifies its quantitative parameters. With the increase of the deformation degree, the perlite component becomes incomplete and transforms at first, to a fractured perlite and then to ferrite, thereby decreasing the volume ratio of perlite. The polarization of the dislocation structure is observed that, nevertheless, does not lead to the internal stresses that induce the specimen fracture.